Concrete may be defined as a binder and water paste, together with aggregate particles mixed in. Aggregates may be defined as fine and coarse aggregates and they occupy 60 to 75% of the volume of concrete. According to the ACI Education Bulletin E1-99 (“Aggregates for Concrete”), the portion of an aggregate passing a 4.75 mm (N° 4) sieve and predominantly retained on the 75 μm (N° 200) sieve is called fine aggregate, and larger aggregate (bigger than 4.75 mm) is called coarse aggregate. Sand is an example of a commonly used fine aggregate, while gravel or crushed stone, in particles larger than 4.75 mm, are considered examples of coarse aggregate.
The properties of aggregates affect the workability, durability, strength, thermal properties and density of the hardened concrete and therefore are a key ingredient in the mix design.
For example, an excess of finer particles (<75 μm), such as silt, clay or fine dust, may impact negatively the quality of the hardened concrete, reducing the overall strength gain in the final product. ASTM C117 standards (Standard Test Method for Materials Finer than 75-μm) preview a maximum limit of 10% for such smaller particles in the fine aggregate, while BS EN 12620:2002 (British Standard—Aggregates for Concrete) establishes a limit of 3% for said components.
When the fine aggregate has a high content of smaller particles, it needs to be washed so that the content of smaller particles is reduced to allowable values, to ensure that the properties of the final product are not compromised. This leads to a high water consumption that at the end of the process is contaminated with the fines removed from the aggregate. This water has to be treated or disposed of, with the obvious environmental problems that this entails.
The alternative to the washing procedure of the fine aggregate is its disposal, since it cannot be used as a construction material, normally by being landfilled, which represents a tremendous waste and also causes an environmental issue.
It has been found that it is possible to convert aggregates, including but not limited to fine aggregates, for example sand or brick dust, into granules with customized physical and mechanical properties, depending on the final application that is desired, such as permeable pavements' subbase or base, decorative purposes or even as total or partial replacement of coarse aggregates in concrete. Furthermore, this invention allows the use of various types of fine aggregates that normally would not meet the requirements of usage in the construction industry and would have to be disposed of, such as waste mud, recycled sand or fine aggregate with high content of smaller particles, such as slit, clay or other impurities, therefore contributing as an alternative to disposal sites and associated constraints (shortage of disposal sites, long transportation distances, law restrictions, etc.).
Furthermore, in case the aggregates are used as aggregates to build a permeable pavement, the final pavement should have low content of fines (passing 0.063 mm no more than 5% and 0.125 mm no more than 20%), so that it has a high proportion of void spaces which will act as reservoirs, gradually releasing the water into the environment over a period of time, therefore reducing the possibility of a flood. The present invention results in aggregates whose size can be predicted, therefore tailored for applications such as the construction of permeable roads.
In addition, in case the aggregates are used as a base for functional roads, the final product should also have good mechanical properties, in order to be able to carry the weight of the layers above itself, as well as the weight of vehicles. Therefore, good resistance against abrasion and fragmentation is needed (Los Angeles test should be maximum 60), which can be achieved using the present method.
The Los Angeles (L.A.) abrasion test is a method to assess how hard an aggregate is and its abrasion properties. These are important because the aggregates used in pavements must resist crushing, degradation and disintegration to ensure the endurance of the future pavement.
The presence method not only works with fine aggregates, but also with bigger aggregated, up to 30 mm, which can be agglomerated to produce gabions or for decorative purposes.
Some applications already disclose the process of turning fine aggregates into a granular material:
DE2110498 relates to a process for preparing grain shape expanded ceramic materials that can be used as aggregates for lightweight concrete. This process encompasses 1) the mixing and homogenization of the starting material with water, 2) forming granules by rolling the material and coating it with a cement layer and 3) drying such granules in an oven at 1000° C.-1200° C. The process does not provide a method to control the physical and mechanical properties of the granules produced; also, the complexity of this process does not allow it to be performed at the jobsite, like the present invention does.
DE3315206 refers to a method to produce sand granules to be used as concrete aggregates, in substitution of the gravel or crushed stone. The sand is agglomerated into bigger spheres with the addition of liquid and optionally with a binder. After, the spheres are added to concrete. The method does not disclose the usage of clayey/silty sand and it shows no results, therefore any indication on how to control the granules' properties and/or the characteristics of the final spheres obtained, thus their applicability is limited or even uncertain.
CN103880371 provides a method for preparing artificial aggregate from sludge and mud with high sand contents. First, the initial material is filtered to remove the particles with high particle diameter; then, it is mixed with binder (cement, and fly ash, and born lime, and slag, and silicon ash), activator, early strength agent and water. Granules are then formed and, after cured, are added to cement as gravel substitute to produce concrete. There is no indication of the final properties of the granules, consequently it does not disclose a method on how to control said properties.
In conclusion, the prior art has not so far disclosed a method to customize the properties of the aggregates produced, in terms of particle size and mechanical performance, tailoring them to diverse applications in the construction industry sector.
The problem to be solved is providing a method to reuse finely divided material even fines that would normally be disposed of, such as fines with high clay content to produce coarser aggregated material.